1. Field of the Invention
This invention relates to system for use in oilfield and pipeline operations to monitor and control asphaltenes precipitation in formation fluids. This invention particularly relates to a system and the associated method for determining whether asphaltenes precipitating out of solution in a wellbore, pipeline and the like are being deposited inside the wellbore.
2. Background of the Art
Many formation fluids such as petroleum fluids contain a large number of components with a very complex composition. For the purposes of the present invention, a formation fluid is the product from an oil well from the time it is produced until it is refined. Some of the components present in a formation fluid, for example wax and asphaltenes, are normally solids under ambient conditions, particularly at ambient temperatures and pressures. Waxes comprise predominantly high molecular weight paraffinic hydrocarbons, i.e. alkanes. Asphaltenes are typically dark brown to black-colored amorphous solids with complex structures and relatively high molecular weight. In addition to carbon and hydrogen in the composition, asphaltenes also can contain nitrogen, oxygen and sulfur species. Typical asphaltenes are known to have some solubilities in the formation fluid itself or in certain solvents like carbon disulfide, but are insoluble in solvents like light naphthas.
When the formation fluid from a subsurface formation comes into contact with a pipe, a valve or other production equipment of a wellbore or when there is a decrease in temperature, pressure, or change of other conditions, asphaltenes may precipitate or separate out of a well stream or the formation fluid while flowing into and through the wellbore to the wellhead. While any asphaltene separation or precipitation is undesirable in and by itself, it is much worse to allow the asphaltene precipitants to accumulate by sticking to the equipment in the wellbore. Any asphaltene precipitants sticking to the wellbore surfaces may narrow pipes; and clog wellbore perforations, various flow valves, and other wellsite and downhole equipment. This may result in wellsite equipment failures. It may also slow down, reduce or even totally prevent the flow of formation fluid into the wellbore and/or out of the wellhead.
Similarly, undetected precipitations and accumulations of asphaltenes in a pipeline for transferring crude oil could result in loss of oil flow and/or equipment failure. Crude oil storage facilities could have maintenance or capacity problems if asphaltene precipitations remain undetected for an extended period of time.
As a result of these potential problems, during oil production in production wells, the drilling of new wells, or work overs of existing wells, many chemicals, also referred to herein as xe2x80x9cadditivesxe2x80x9d, including solvents, are often injected from a surface source into the wells to treat the formation fluids flowing through such wells to prevent or control the precipitation of asphaltenes. In addition to controlling asphaltene precipitations, additives are also injected into producing wells to, among other things, enhance production through the wellbore, lubricate downhole equipment, or to control corrosion, scale, paraffin, emulsion and hydrates.
All these chemicals or additives are usually injected through a conduit or tubing that is run from the surface to a known depth. Also, chemicals are introduced in connection with electrical submersible pumps, as shown for example in U.S. Pat. No. 4,582,131 assigned to the assignee hereof and incorporated herein by reference, or through an auxiliary line associated with a cable used with the electrical submersible pump, such as shown in U.S. Pat. No. 5,528,824 assigned to the assignee hereof and incorporated herein by reference.
While much more commonly used to minimize paraffin deposition problems, it has been disclosed that asphaltene precipitation can be, if not controlled, at least mitigated by providing heat to equipment to raise the temperature of crude oil, for example, to a temperature higher than its cloud point, also referred to as the deposition temperature, to prevent or at least minimize asphaltene precipitations. A circulating heat transfer fluid or medium is usually used as the heating means to effect the desired temperature changes.
Several other ways of addressing the asphaltene precipitation problems are also known. For example, U.S. Pat. No. 5,927,307 discloses an apparatus for environmentally acceptable cleaning of oil well components including removing paraffin and asphaltenes from the rods of the rod string of an oil well. U.S. Pat. No. 5,795,850 discloses an oil and gas well operation fluid used for the solvation of waxes and asphaltenes, and the method of use thereof. U.S. Pat. No. 5,827,952 discloses an acoustic-wave sensor apparatus and method for analyzing a fluid having constituents, which form deposits on the sensor when the sensor is cooled below a deposition-point temperature.
Whether performing a specific mitigating, remedial or prophylactic treatment or measuring a particular property of the formation fluid, these disclosed methods are typically indirect and involve one or more steps handled by an operator manually. Some of these methods are not very sensitive or require time-consuming measurements or analysis in a laboratory. Alternatively, where automated analytical methods are disclosed, such as in U.S. Pat. No. 6,087,662, the methods require prohibitively expensive apparatus and are complex and difficult to apply to a field application. Consequently, it is difficult and sometimes not feasible to automate the process of monitoring and controlling asphaltenes at a wellsite or in a pipeline system.
Another problem with trying to control asphaltene precipitation with conventional methods is that the cycle time is normally quite long between the times the samples are collected, the measurements are performed and, if needed, any adjustments of a particular treatment are made. As a result of this long cycle time, it is possible and even likely that either too much additive is added for unnecessary and expensive over-treatment, or too little is added for under-treatment, resulting in either wasted chemicals or excessive undesirable asphaltene precipitations or separations from the formation fluid. The same problem exists when the temperature of the formation fluid is used to control asphaltene precipitations and separations. Either under heating or overheating of a piece of equipment that may take place at an oil well or a pipeline system, resulting in inadequate heating or unnecessary waste of energy.
The present invention provides a system that uses one or more sensors to measure, directly and in real time at the wellsite or in a pipeline, a relative concentration of asphaltenes in a formation fluid or crude oil. The present invention also provides a system that measures the difference in relative asphaltene concentration in the formation fluid retrieved at the wellhead and that entering the wellbore from the formation. If the difference is larger than a predetermined range, a signal is transmitted from a controller or control unit to an apparatus to adjust the treatment relating to suppressing, controlling, inhibiting or otherwise mitigating asphaltene precipitations. It is also envisioned that the present invention may be used for monitoring asphaltenes in pipelines transporting oil from one location to another and controlling the necessary treatments.
In one aspect, the present invention is a system for determining the relative concentration of asphaltenes in a formation fluid from direct on-site measurements made on the formation fluid recovered from a subsurface formation, comprising: a fluid flow path for flowing formation fluid recovered from a subsurface formation; a sensor associated with the formation fluid in the fluid flow path providing data corresponding to the relative concentration of asphaltenes in the formation fluid in the fluid flow path; and a processor for determining from the data the relative concentration of asphaltenes in the formation fluid.
In another aspect, the present invention is a method for monitoring the relative concentration of asphaltenes in a formation fluid comprising the steps of: determining a relative concentration of asphaltenes in a formation fluid passing through a fluid flow path for recovering the formation fluid from a subsurface formation; making a subsequent determination of the relative concentration of asphaltenes in the formation fluid; and comparing the relative concentrations of asphaltenes in the formation fluid; wherein the determinations of the relative concentration of asphaltenes in the formation fluid is done on site, using a processor, in real time or near real time.
In still another aspect, the present invention is a method for monitoring and controlling the precipitation of asphaltenes out of a formation fluid comprising the steps of determining a relative concentration of asphaltenes in a formation fluid passing through a fluid flow path for recovering the formation fluid from a subsurface formation; making a subsequent determination of the relative concentration of asphaltenes in the formation fluid; and comparing the relative concentrations of asphaltenes in the formation fluid; wherein the determinations of the relative concentration of asphaltenes in the formation fluid is done on site, using a processor, in real time or near real time and additionally comprising pumping additives into the formation fluid when the difference in the relative concentrations of asphaltenes in the formation fluid is outside of a predetermined range.